1. mission; candidate engines range in development from pulsed operation at the appropriate power level or steady state operation at lower power levels, to engines requiring proof of concept. The workshop review panels concluded that compelling benefits would accrue from the development of nuclear electric propulsion systems, and that a focused, well-funded program Is required to prepare the technologies to support the missions of interest" (Barnett 1991). Nuclear Thermal Propulsion
2. The basic features of a nuclear thermal propulsion (NTP) system are shown in Figure 9 which is based on the solid-core reactor (SCR) system. Basically, as shown in Figure 10 for a SCR, the NTP uses a nuclear reactor to heat the working fluid (usually hydrogen) directly to very high temperatures (-2300 K to -3100 K for SCRs and higher for Iiquidcore [-5500 K] and gas-core reactors [up to -10 OOOK)); a nozzle through which the hot hydrogen expands; and a turbopump to force the hydrogen through the NTP system. The nonnuclear components (e.g., nozzle, turbopumps, valves, piping, tanking) are similar to the components on chemical propulsion systems except that they must be designed to operate in a high radiation environment (Spence 1968).
3. As Roderick W. Spence has noted: "If you've ever done much work with hot tungsten filaments, you know that a 20000C filament is mighty bright - too bright to look at without smoked glasses. And 25000C is really dazzling. Now, instead of a tiny filament, imagine a mass of material about the size and shape of an oil drum, and imagine that some internal energy source only slightly less powerful than Hoover Dam is heating the drum to an average temperature close to 2000oC. Imagine further that cold hydrogen is being pumped into one end of this incandescent mass at a rate of 70 Ib/sec, and hot hydrogen at a temperature of 20000C is scorching out the other end. You are now imagining a nuclear rocket engine In operation" (Spence 1965).
4. Spence has written: "The sole reason for developing nuclear rockets is that they can produce higher exhaust velocities (by a factor of two) than the best chemical rockets. They can do this . because the propellant and the energy to propel it come from separate sources. The heat (which comes from the reactor) heats the propellant (which comes from a tank) and drives it out the exhaust nozzle. In general, the exhaust-gas temperature in the nuclear rocket is no higher than it is for chemical rockets but in nuclear rockets the propellant can be chosen to minimize molecular weight. Hydrogen with a molecular weight of 2 is al 1f!obvious choice despite its low density (which leads to large tanks of propellant). It is this choice that gives the nuclear rocket an exhaust velocity of about 8 kmlsec" which as Spence has noted compares "to about 4 kilometers per second for the best velocity obtained from chemical rockets" (Spence 1965 and 1968).